Advanced Trees for heating and cooling

There are many advantages to creating a green oasis around the home. Man-made structures alter micro climates. Trees can make a difference.

Generally speaking hard surfaces such as concrete, asphalt and brick heat more rapidly and retain more heat than grass and treed areas. The entire surface area of our urban environment is absorbing energy and releasing heat. Air in these places is heated simply by being in contact with these surfaces. Around the home, rainfall is not absorbed by hard surfaces as it is quickly removed by gutters and drains thereby robbing us of a natural cooling mechanism. Planting trees and shrubs around your home can cool an area and modify the rate at which energy is exchanged.

So where do you start? Tree placement is your first consideration. Where are you looking to plant trees and shrubs? And by planting these trees and shrubs, what are you trying to achieve? There are a number of external considerations that may impact the choices you make at this point.

• Which way does your home face? Do windows and doors face north, south, east or west?
• When and where does the sun hit the house?
• Where around the house is the cool summer breeze felt?
• Are there rooms that get too hot in summer?
• What are the outside areas for?
• Are there hard surface areas close to the house?

Answers to these questions provide the scope of the project and essentially encapsulate what you are trying to achieve with regard to defining tree heights, tree widths and proximity to existing structures, duration of daylight and position of competing trees.

Most situations benefit from an understanding of two key propositions: that deciduous trees shed leaves once a year and evergreen trees remain full and flush for the full 12 months of the year. Understanding the basics such as the difference between deciduous and evergreen, coupled with knowing how the sun travels over your property (the external considerations) are paramount to good landscape design. Matching tree with purpose then becomes a matter of common sense.

Deciduous trees are ideal for windows facing north, north-east and north-west. Those trees that shed leaves annually are deciduous and due to their capacity to be full in the hottest parts of the year and bare in the coldest, they are best placed in these positions. Letting in winter light while sheltering the house from the hottest summer rays, is the ultimate win-win scenario.

Unshaded paving to the north, east and west should be avoided as hard surfaces reflect heat upwards into windows. Lawns and low ground covers are a good option for these areas and if paving is unavoidable say in the case of an existing driveway, deciduous trees that offer good shade and wide canopy can alleviate the problem.

Our recommendation is for small or medium-sized trees, typically between 6-12 metres tall with good open branching to achieve horizontal reach. Gingko biloba is an excellent choice with horizontal branching and wide leaves. It offers attractive clean pale limbs during winter and a healthy grassy green foliage colour in its summertime glory. Or for striking effect try Robinia or Gleditsia. Both are open, summer trees for shade. They also do a marvellous job cooling concrete and timber with the added bonus of an eye-catching lime green leaf that looks fabulous offsetting grey bitumen, white timber and red brick.

Due to the direction of the summer sun and its position higher in our sky, east and west facing windows are difficult to protect from harsh summer heat. Deciduous trees and vines on trellis can assist here but you will need to invest in outside shading devices such as external blinds and shutters as the harshest light is horizontal early in the day on the east side of the house and at late afternoon on the west side. Only vertical screening can block the sun at these angles.

South facing windows are a little more difficult to shield from harsh summer sun. With no direct winter sun but with at least 8 hours of sunlight positioned directly above the house in summer, landscapes need to cater for two distinct situations; landscapes that allow as much light as possible into the house during winter and almost total block-out in summer.

Tall evergreens placed to the south-east and south west will help control the harsh angular summer rays that heat the house at the ends of the day in summer whilst those in-between times, where the sun’s rays remain constant above the house, deciduous trees planted in close proximity to the house can give much needed shade relief, particularly if your outdoor entertaining occupies this aspect.

A full day of analysing how the sun travels over your property in summertime particularly from the south side will benefit you enormously and ensure that trees can be placed in the most strategic of positions to alleviate the brash sun heating your home when you need cool respite.

Good deciduous specimen trees work really well here as they are attractive to view and provide natural shelter from the sun. In addition to Gingko and Robinia already mentioned, there is Pistachia chinensis  (Chinese Pistachio) and Ulmus parvifolia  (Chinese Elm) both with their attractive green finer green foliage or Pyrus ussuriensis  for its attractive pointed leaves and open habit.

Place evergreen trees for wind protection. Evergreen trees are full and flush and remain that way all year round. A solid placement to the south can also assist cooling in summer by directing south-westerly sea breezes into and through the home. Good examples to consider include Magnolia ‘Little Gem' or ‘Exmouth’ due to their broad leaf and dense habitat. 

If a native screen is required, we recommend a storey planting with either two or three levels of trees and shrubs. In this instance try Eucalyptus melliodora or cladocalyx 'Nana' underplanted with grevilleas and/or banksias. Similarly these same evergreens shrubs and trees planted to the west and north-west can shield your home from winter storms. Be mindful that the trees and shrubs you choose need to be considerate of both planting and growing space and proximity to existing structures and utilities.

This is the kind of ‘green’ attitude we all must employ to make a long-term environmental difference. Trees can make a difference.